In the prior art one method of making thermoplastic film is to extrude the thermoplastic material in the shape of a tube with relatively thick walls, cool and flatten the tube, and subsequently heat and expand the tubing with internal air pressure to stretch the film to the desired thinness and/or orient the film. The relatively thick-walled tubing in the unexpanded condition is often referred to as "tape" in the art.
When a thermoplastic material leaves the extruder die it is in a molten and non-self-supporting condition. When attempts are made to increase the speed of extrusion, the molten extrudate will neck down unpredictably and stretch non-uniformly under its own weight before it solidifies. One prior art method used to overcome this problem is to employ a coaxially mounted spreader and former with a roughened surface over which the non-self-supporting tubular film is passed in contact therewith. In combination with the former an after baffle is used having the same diameter as the former in order to assure that the pulling force around all sides of the tubular extrudate is the same. Such a method and apparatus is described in U.S. Pat. No. 3,084,386 which issued to M. R. Gerow on Apr. 9, 1963. However, it has been found that contact with a spreader or former prior to solidification of the interior wall of the tubular extrudate will result in undesirable irregularities in the interior surface of the tube. Accordingly, it is one object of the present invention to provide a tubular substrate with a substantially uniform, unscuffed interior surface.
Another object of the subject invention is to provide a process and apparatus which will allow high speed production of a tubular extrudate.
Still another object of the subject invention is to provide a process and apparatus wherein the extrudate remains stable as it is taken from the extrusion die.
These and other objects of the subject invention will become apparent to those skilled in the art from the following Summary of the Invention. Drawings, and Detailed Description of the Invention.